The present invention relates to light referencing methods, and in particular, to a method for referencing the wavelength of a tunable laser.
Tunable lasers which operate in the visible or ultarviolet light region are used in a variety of experimental and commercial settings. Often it is important to operate a tunable laser at a precisely known beam wavelength. One prior art method for setting (and holding) a laser beam at a known wavelength is to reference the beam, in a spectrometer, with a known spectral line wavelength produced by an atomic emission lamp. A disadvantage of this method is that the requisite high-quality spectrometer and the associated photomultiplier tubes are quite expensive, as are the reference atomic emission lamp and its associated power supply.
It has also been proposed, as a method of referencing a laser light beam, to use the laser beam directly as an exciting light source to produce excited states in free metal atoms at known spectral-line wavelengths associated with the atoms. The use of atomic reference standards, as opposed to molecular standards, allows the user to trace wavelength calibration back to international standards. This approach has been limited heretofore by the problems inherent in producing a vapor of most transition metal elements from a solid sample of the metal.
It is therefore one general object of the present invention to provide a laser referencing technique which overcomes above-discussed problems associated with laser wavelength referencing techniques known in the prior art.
A more specific object of the invention is to provide a simple, relatively inexpensive method for referencing the light frequency of a tunable laser with a known spectral line wavelength of a free transition metal atom. Another object of the invention is to provide such a method which is usable for a referencing laser beam wavelength in both visible and ultraviolet light ranges.
It is a further object of the invention to provide such a method which can be practiced with a wide range of transition metal atoms.
In the method of the invention, a voltage is placed across a pair of electrodes in a cell containing vapor of an organometallic compound containing the desired transition metal atom. The vapor is irradiated with a non-saturating pulse of focused laser light, that is, a pulse having an average flux density level at which electronic-excitation transitions of the free metal atoms are wavelength dependent. The pulse produces multiphoton dissociation of the organometallic compound, resulting in free metal atoms in the vapor. A population of these atoms become photoionized through a multiphoton process that involves an excited state of the metal atom. The laser wavelength is adjusted until a peak current flow across the electrodes is observed, evidencing maximum photoionization which occurs when the laser wavelength is set at a spectral line of the free metal atom.